Inhalation studies are generally performed using inhalant systems. These studies are typically performed by exposing one or more animals to either an organic or inorganic inhalant inside an inhalant chamber. Various implementations of such a system maintain the chamber environment using a proportional integral derivative (PID) controller and measurements of pressure, temperature, humidity, air flow into and air flow out of the chamber. In some inhalation systems, the animal's respiratory volume and respiratory rate can be calculated from inhalant chamber measurements such as temperature, humidity and pressure variations. The individual breath dose can then be determined by multiplying the average inhalant concentration during each breath by the calculated respiratory volume for that same breath. The accumulated dose for the entire exposure may then be determined by summing the product of the individual breath volumes and its average inhalant concentration.
Alternatively, direct, near real-time measurements of animal respiratory parameters may be made under conditions similar to those during the inhalation study some time prior to the actual study. These measurements may then be multiplied by the average inhalant concentration to infer an individual breath dose and ultimately the accumulated dose during exposure.
Respiratory volume calculations using chamber measurements are susceptible to errors. Chamber baseline pressure changes may occur due to differing degrees of neck seal of test animals. Chamber pressure may also vary when a nebulizer used to introduce the inhalant into the chamber switches on and off as it regulates the inhalant concentration. These non-respiratory related variations in baseline pressure have a large influence on the measured pressure variations and therefore on the calculated respiratory volume.
The equations used to calculate respiratory volume from pressure variations present another potential error source. Assumptions that may or may not be valid during the inhalation study may be made for each equation. For example, the ideal gas law neglects molecular size and intermolecular attractions and thus is most accurate for a monoatomic gas at high temperatures and low pressures. By definition, the nebulizer is creating an inhalant which is non-monoatomic therefore inaccuracies are introduced.
Direct near real-time measurements of animal respiratory patterns prior to the actual study avoid the inaccuracies associated with calculated respiratory parameters using inhalation chamber measurements; however, this method is not without its own error sources. Animal respiratory patterns may change significantly throughout the course of a study. Alterations in respiratory rate, volume or both may occur as a result of metabolic demand, level of anesthesia, if used in the study, or the amount or type of inhalant used. These respiratory changes can create large levels of uncertainty with respect to dosimetry. This uncertainty may result in variable results and ultimately poor reproducibility of scientific experiments.
Direct respiratory measurements using a breath analyzer such as a pneumotachograph can provide real time respiratory measurements and thus improve dosimetry accuracy; however, these measurements are often impractical because inhalants often clog the breath analyzer screens. Additionally, many inhalant compounds are biohazards; thus, thorough cleaning and/or decontamination of the breath analyzer is required. This can be a difficult and lengthy procedure that can potentially damage or degrade the highly sensitive equipment.